Will Hubble leave an heir?

This artwork shows one of the possible designs for the Advanced Technology Large-Aperture Space Telescope, which would have more than 40 times Hubble's sensitivity.

When Hubble finally fades into the sunset, what will take its place? More space telescopes are on tap, but some question whether any of them can truly replace the grand old observatory.

If you were to ask the scientists on the Hubble Space Telescope's team whether there's a successor, they'd have a quick answer: the James Webb Space Telescope, which is destined for launch in the 2013 time frame, just when Hubble is expected to wrap up its work for good.

NASA, the European Space Agency and the Canadian Space Agency are spending $3.5 billion or more on the telescope, which is destined to observe the cosmos from a gravitational balance point about 1 million miles from Earth, known as L2. It's designed to take the handoff from Hubble as NASA's greatest observatory, and I've referred to it more than once as "Hubble's Heir."

Zimmerman's point is that the Webb telescope is optimized for a set of wavelengths that are different from Hubble's sweet spot. Hubble's best observing takes place in the visible-light spectrum, the kind of light we can see with our naked eyes. In contrast, Webb will focus on infrared wavelengths, which are off the red end of the rainbow.

Astronomers say infrared telescopes provide the best way to see objects at the edge of the cosmos, because wavelengths at that distance are redshifted due to the expansion of the universe. But Zimmerman argues that false-color infrared images won't capture the public's attention the way Hubble's images have. Even though Hubble's pictures are enhanced during processing to boost the colors, the visible-light view serves as the basic framework.

"It's not that optical astronomy is the only thing - the end-all, be-all," he said. "But without the optical, it's difficult for the human brain to synthesize all the other data."

The trend in telescopes is to send up spacecraft that have been optimized for specific tasks rather than general observing. For example, NASA's Kepler probe, which had its scientific coming-out party today, is a telescope expressly designed to look for extrasolar planets.

The European Space Agency's Herschel and Planck telescopes, due for launch on Thursday, are designed to corral the infrared and microwave wavelengths, respectively. Like the James Webb telescope, Herschel and Planck are destined to hang out at the L2 point. Like Webb and Spitzer, Herschel will peer toward the far reaches of the observable universe and look into the dusty cradles of stars and planets. Like the Wilkinson Microwave Anisotropy Probe, Planck will look for the imprint of the big bang on cosmic background radiation.

The new instruments will bring capabilities that the old instruments never had, but they're not nearly as versatile as Hubble, Zimmerman said: "Although the focused research telescopes are useful and necessary, general observatories are useful and necessary as well."

Astronomers always have plenty of big ideas for future observations, and right now a host of proposals for next-generation space telescopes are being considered as part of the National Academies' Astronomy and Astrophysics Decadal Survey. Among the concepts are the New Worlds Observer, which would seek to characterize alien worlds; and the Advanced Technology Large-Aperture Telescope (a.k.a. ATLAS Telescope), which would have more than 40 times Hubble's sensitivity.

The results of 19 NASA-funded next-generation studies - funded at levels ranging from $250,000 to $1 million - are expected to be released later this year, once the astronomers involved in the Decadal Survey have reviewed them. But the telescopes won't be launched until well after the Webb begins operations.

By then, the debate over whether to let Hubble die could well be raging once again. One of the tasks to be accomplished during a series of Atlantis spacewalks is the installation of a docking device that a future robotic craft could hook onto - in order to drive Hubble out of orbit to a controlled, fiery re-entry.

Zimmerman points out that the same docking device could conceivably accommodate spaceships carrying the supplies and/or the personnel for yet another servicing mission. Might Hubble's best successor in 2014 be ... Hubble itself?

"If Hubble is still functioning, and there is no replacement, I'm willing to bet NASA will not be talking about deorbiting it. They'll be talking about keeping it alive," Zimmerman said. "This is the telescope that refuses to die."